This invention relates to a catalyst support and a process for the preparation thereof. In particular, this invention relates to a silica/alumina catalyst support which can be used with a catalytically active component to catalyst a variety of reactions, particularly but not exclusively hydrogenation reactions.
A wide variety of catalyst supports are known including those based on porous refractory oxides, such as silica. The use of silica gel support normally produces relatively poor results.
It is known to prepare amorphous silica-aluminas from aluminium salts such as aluminium chloride or aluminium sulfate and silicon sources such as sodium silicate (waterglass). Such a method is for example described in U.S. Pat. No. 4,988,659. The main disadvantages of such amorphous silica-aluminas, is the necessity to spray-dry the cogel mass to form spherical particles, and a high residual salt content, irrespective of the washing steps. Indeed, many anions such as Cl− and/or (SO4)2− remain trapped in the silica-alumina structure, even after washing, while the corresponding cations, usually Na+, are fixed into the silica-alumina structure.
Various documents have disclosed the combination of silica gel and alumina and these include U.S. Pat. No. 2,285,314 where silica gel is mixed with hydrous alumina to form a silica/alumina catalyst, and GB 1,223,878 where silica gel is mixed with an alumina source, such as sodium aluminate, at a pH of between 7.5 and 11 to produce a silica support with alumina particles precipitated thereon. These supports both result in a limited improvement in performance over pure silica.
Various other documents, such as GB 1,178,594, have disclosed the use of synthetic zeolites (alumino-silicate) as catalyst supports. Synthetic zeolites are crystalline solids. Synthetic zeolites are usually formed by a process of slow crystallization of a silica-alumina gel (obtained from finely divided pure silica and sodium aluminate) in the presence of alkalis and organic templates, followed by washing and spray drying. One of the important processes to carry out zeolite synthesis is sol-gel processing.
Zeolites give better results as catalyst support but are expensive to manufacture. Furthermore, synthetic zeolites usually consist of small particles (around 10 μm ). As a catalyst support usually has a size of around 100 μm, the zeolite particles (around 10 μm) must be agglomerated in bigger particles (around 100 μm), which is usually carried on using a binder. Such products have the disadvantage of being more sensitive to attrition compared to “one piece” products, such as those based on silica gel.